The present invention relates to an improvement in oil well tools known to the art as hydraulic bumper jars, of the type including a hammer and anvil, in which the hammer is retarded when urged towards the anvil in order to tension the drill string, and then released in order to induce a violent impact of the hammer on the anvil, which impact is transmitted to the down-hole tool members suspended below the jar within the well bore.
As is well known to those skilled in the art, well jars are connected to down-hole equipment which has become stuck in the well bore, such as drill bits, twisted-off drill pipe and other objects that have accidentally become lodged within the hole due to sloughing rock, caving or other debris present within the bore hole. Devices of the class to which this invention relates are used for the purpose of impacting the lodged or stuck member in an effort to dislodge it for removal. Impact blows may be delivered to the lodged member, known as a "fish", in a downward direction, by dropping the weight of the drill string on the fish, in an attempt to drive it downwardly, and such an operation is known as "bumping"; when the impact is delivered in an upward direction, the operation is known as "jarring" and it is with this latter operation that the device of the present invention is primarily concerned.
Jarring tools may be either mechanical or hydraulic, the upwardblow of the hammer on the anvil being released by mechanical means or hydraulic means, respectively; the hydraulic jar is now most common in the industry.
A hydraulic jar comprises an outer tubular housing portion including the anvil, and an inner mandrel portion including the hammer, which portions are attached, respectively, to the bit and the drill string by means of threaded attaching components known as "subs". The mandrel portion slides axially within the housing portion in telescopic fashion, the two components being connected by means of splines, so that torque can be transmitted between them while permitting longitudinal relative movement. Means are also provided to limit the extent of longitudinal relative movement of the mandrel and outer housing, so that they cannot separate from one another.
The jar tool is normally used adjacent the bottom of a well bore which, during the drilling operation will contain a column of heavy mud which is used to create hydrostatic pressure against the walls of the well bore, in order to prevent the uncontrolled release of hydrocarbon fluids from the rock formations penetrated, a condition known as a "blow out". The jar tool must be designed to operate under down-hole conditions of high pressure, high temperature, abrasion, heavy loading in both tension and compression, and high torques.
Previous jars of the type to which this invention relate have necessarily been run in "closed" condition, in order to prevent the open spline drive, which would normally be exposed to the mud, from filling with rock cuttings. There is a distinct drilling advantage to running the jar in open condition, this arising from the necessity that when adding another length of drill pipe to the string, the string must be raised off bottom and held in slips. A hydraulic jar if run in closed condition in such circumstances will frequently activate itself from the weight of the drill string hanging below it. This unexpected jarring action may cause the drill string to release from the slips, and fall down the well bore, creating an expensive fishing job. With the jar that is run in open position, this problem is avoided.